On Thu, 2010-06-24 at 15:02 +0900, Yoshiaki Tamura wrote: > > Hi Alex, > > Is there additional overhead to save rams introduce by this series? > If so, how much?
Yes, there is overhead, but it's typically quite small. If I migrate a 1G VM immediately after I boot to a login prompt (lots of zero pages), I get an overhead of 0.000076%. That's only 226 extra bytes over the 297164995 bytes otherwise transferred. If I build a kernel on the guest and migrate during the compilation, the overhead is 0.000019%. The overhead is tiny largely due to patch 12/15, which avoids sending the block name if we're working within the same block as sent previously. If I disable this optimization, the overhead goes up to 0.93% after boot and 0.26% during a kernel compile. Note that an x86 VM does a separate qemu_ram_alloc for memory above 4G, which means in bigger VMs we may end up needing to resend the ramblock name once in a while as we bounce between above and below 4G. Worst case for this could match the 0.26% above, but in my testing during a kernel compile, this seems to increase the overhead to 0.000026% on a 6G VM. I don't see any reason why we couldn't allocate all the ram in a single qemu_ram_alloc call, so I'll add another patch to make that change (which will also shorten the name to "pc.ram" for even less overhead ;). Thanks, Alex
